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Try systemctl --user restart pipewire pipewire-pulse

I really don’t understand dbus.

I think systemd targets work opposite to your expectation. The Wants in [unit] define the things that that unit needs to already be available. For instance, you might add Wants=network.target to the unit for nginx so that it won’t try to start until the network is available. When I wrote a unit to start my company’s application, I also had Wants=postgresql.service to ensure that the database came up before the application. Remember that sysyemd tries to run as many things in parallel as it can. This is one thing that makes it much faster than classic sysvinit which started things sequentially. But it means race conditions can occur. You use Wants to break those races where necessary. The targets that you’d specify in WantedBy in [install] more closely resemble SysV runlevels. You might want to read how runlevels used to work in SysV, in order to understand systemd targets.

Every user can enable services from /etc/systemd/user for their account. If the user doesn’t log in, their instance of the service won’t start. There is a way to have user services launch without logging in, but that would obviously be nonsensical for desktop services.

I don’t think systemd would find units in /etc/systemd/user/KDE. Look at the mess that is /usr/lib/systemd/system. Organization doesn’t seem to be a thing.

For your unit files, you have Wants in the [Install] section. That is not correct. Wants belong in the [Unit] section. The [Install] section is where you define WantedBys. You may want to read the man page for systemd.unit.

To interact with user services, you do have to always use systemctl --user.

If you put your user unit files in /etc/systemd/user, they’re accessible to all users. If a particular user wants to enable the service, they can run systemctl --user enable $service. Defining the unit in ~/.config/systemd will mean only the one user will be able to start the service. Defining the unit in /etc/systemd/system indicates it is not a user service but a system service.